Leap of Faith: Blue Screens Explained

Movie viewers know that an actor cannot swing like a spider from a skyscraper or converse with an animated rabbit, but visual-effects artists make such scenes believable. The technique they exploit is the matte process—commonly called blue screen or green screen for films and chroma-key for television.

The process, pioneered in the late 1930s, remained largely unchanged for decades. An actor was filmed onstage in front of a blue or green drape or wall, then a different background scene was filmed. Technicians masked out the drape color, made positive and negative transparencies, physically overlaid the strips and projected them onto fresh film—creating the final composite scene. This “optical printing” exercise was tedious and costly but effective. Another process was later developed for television broadcasting.

A backdrop could be any color—but because red, green and blue correspond to film’s three emulsion layers and to a television camera’s three color channels, they are easiest to filter. Red is common in skin tones, however, and masking it out complicates skin’s appearance, so that color is often avoided.

Only in the mid-1990s did a fully computerized process begin to take over, in which software converts film frames to digital files and allows artists to manipulate them [see illustration above]. Moviemakers have rapidly embraced the technique because it is faster, cheaper and more refined. Most feature films today are assembled digitally.

Human editors must still conduct the procedure, however, adjusting shadows, correcting colors and fine-tuning outlines so viewers will not perceive fleeting apparitions or artifacts. “Despite the technology, the process is still an art form; it still requires the human touch,” says Chris Cookson, chief technology officer for Warner Bros. Entertainment in Burbank, Calif.

Success at major studios has spawned simpler matte programs for home and digital video buffs, such as Final Cut 6 and Avid Xpress, which have improved quickly. The products are “almost like getting a $30,000 editing and special-effects system in a $1,200 software package,” says Walter Graff, a director and cinematographer in New York City. Whether an enthusiast has the eye and skill to exploit that package like a professional is another story.

Did You Know
THEN AND NOW: Movie viewers who would like to see how software has improved matting can compare the original Star Wars: The Empire Strikes Back (1980) and the 2004 DVD version, which was remastered using more refined techniques; both are included in the 2006 Limited Edition DVD of the film. The HD DVD of the film 300 includes a bonus feature showing how extensive blue screening was done.

WITNESS: Software sometimes has difficulty cleanly matting complex backgrounds with actors who are moving quickly across a blue or green screen. Technicians may paste tennis balls or bright dots in a pattern on the screen to act as reference or “witness” marks that can help editors fine-tune the tracking. The mark color must also be matted out. The aid was used extensively in Space Jam, the 1996 basketball romp starring Michael Jordan and Bugs Bunny.

Blue-screen Notables1940 - Thief of Bagdad, first use of manual blue-screen technique

1963 - Jason and the Argonauts, sophisticated combination of live actors and stop-motion animation